What are the forces on a tied down glider?

Derek Copeland wrote:
I have heard of a case where a Blanik was wrecked by a violent gust of wind
during a storm, even though it was very well tied down. The lift on the
wings was sufficient to lift the glider. The wingtips were both tied down
and the mainspar was damaged beyond economic repair by the resulting
downwards bending loads!

That might have been our club's Blanik in the '80s. In retrospect, it
wasn't "very well tied down". Only the tips were tied down well to solid
ground anchors; the fuselage was not restrained directly, and the center
of the wing, using the factory tie down rings, was tied to a cable that
could lift a foot or so with 100-200 pounds of force.

It was that experience that makes me question a lot of the tie down
methods I see that use just the wing tips. I'm now of the opinion the
best situation has the fuselage restrained using the towhook, or perhaps
the landing gear. If that straps and ground anchor can take 5+ Gs, it
doesn't matter much how well the wings are restrained. The tail
restraint is probably important if very strong quartering winds are
encountered.

--
Eric Greenwell - Washington State, USA
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